DE 198 35 108 A1 describes a solid-state laser in which several disk laser crystals are disposed in a common resonator. The laser radiation field penetrates the laser crystals. The resonator includes two end mirrors and additional deflecting mirrors (plane mirrors) disposed between the disk laser crystals to form the laser radiation field.
A resonator for a high-performance solid-state laser is designed to achieve a high resonator stability and constant radiation properties with varying operating parameters. In contrast to conventional high-performance rod lasers, in a high-performance disk laser, the thermally induced spherical wavefront deformation of the resonator radiation field (“thermal lens”) during passage of the laser radiation through the laser-active medium is not the dominating factor for the stability of the resonator when a sufficiently rigid heat sink or a transparent support body is used to stabilize the laser-active medium mounted thereon. A high-performance disk laser can therefore be designed in such a manner that, within the entire pump capacity range, the resonator remains far away from the critical points or limits of the dynamic stability range as described by the stability diagram (g-diagram). This also applies if a resonator includes several laser-active media (disk laser crystals).
Two other effects have a substantially larger influence on the properties of the laser radiation field of a disk laser or a disk laser amplifier. On the one hand, a static wavefront deformation is produced during passage of the laser radiation field through a laser-active medium, and on the other hand, there may arise thermally induced diffraction losses.
The static wavefront deformation of the laser radiation field is due to a deviation of the laser-active medium from its ideal form, and this deviation results mainly from production tolerances. The static wavefront deformation is made of a spherical portion resulting from the deviation of the laser-active medium from the desired radius and an irregular (aspherical) portion. The spherical portion results from the difference between desired and actual refractive power of the laser-active medium and is also called (static) refractive error. The static refractive error adds to the dynamic thermal lens that is present also in high-performance disk lasers, and in some cases, the static refractive error may even exceed the dynamic thermal lens. The refractive errors of several crystals in a disk laser should be taken into consideration in the design of the laser amplifier or resonator. Typically, a laser amplifier should have laser radiation fields of the same diameter on the different crystals, assuming identical refractive power of the crystals. Different refractive powers of the crystals lead to more or less varying beam diameters on the crystals and such variation causes the following problems. The outer regions of the laser radiation field are not amplified on crystals with enlarged beam diameter but are partially absorbed. In contrast thereto, the power available in the outer region of the pump spot of crystals having a reduced beam diameter is not retrieved. Under unfavorable conditions, this can cause local overheating and ultimately destruction of the crystal. In any case, the laser efficiency is reduced.
The thermally induced diffraction losses in the disk laser amplifier are partially due to wavefront deformation of the laser radiation field during passage through the thermally loaded laser-active medium. In the disk laser, the wavefront deformation dominates in the edge region of the pump spot, that is, in the transition region between the hot, pumped crystal region and the cool, non-pumped outer region. The thermally induced diffraction losses in the disk laser amplifier may be due to inhomogeneities of the pump light distribution and distortions of the laser radiation field due to fluctuating regions of different thermal load in the laser-active medium. Moreover, because the wavefront is deformed, the energy in the resonator is redistributed between the resonator modes, causing energy transfer to “leak modes” that are not sufficiently amplified in the amplifier. The disk laser is particularly sensitive to such losses due to its low amplification. The losses thereby depend on the design of the laser and, in particular, on the required beam quality of the laser output radiation. If there is insufficient blocking of diffracted beam parts between the amplifier stages, the thermally induced diffraction effects in the disk laser or amplifier result in steadily increasing deterioration of the beam quality. Subsequent improvement of the beam quality through hard apertures may result in considerable performance or efficiency loss.
The larger the requirements for the beam quality of a disk laser or amplifier are, the larger is the influence of diffraction effects on the efficiency. Additionally, dynamic and static refractive errors have a greater influence on the efficiency as the requirements for the beam quality of the disk laser or amplifier increase. The dynamic stability range, that is, the admissible refractive power range of the laser-active media, is reduced as the pump spot diameter is increased and as the required beam quality is increased.
The influence of the static refractive error and of the thermally induced diffraction losses increases with the number of passages of the laser radiation field through a laser-active medium during a resonator circulation or within an amplifier chain. The effect of the number of passages depends substantially on the design of the amplifier.
The static refractive errors and thermally-induced diffraction losses also influence the efficiency and the resonator stability of high-performance rod lasers with several rods in a laser resonator. This influence can be noticed in particular if the beam quality must be high, for large beam cross-sections, low amplification per passage through a laser crystal, and in the absence of hard apertures (for example, rod surfaces). For dynamically stable rod laser systems with strong thermal lens and high amplification (for example, Nd:YAG or Nd:YVO4 at 1064 nm), the beam quality is limited by the thermal lens and the diffraction losses are typically dominated by the hard apertures such that the thermally induced diffraction losses play a minor role. Only for rod laser systems with a limited dynamic stability range (that is, almost constant pumping power) and no hard apertures and preferably for laser-active media with low amplification (for example, Yb:YAG), the thermally induced diffraction losses can have a substantial influence on the efficiency.
One characteristic property of a disk laser is the absence of hard apertures directly bordering the active volume. The use of hard apertures to determine the beam parameter product in a disk laser is not required and generally not useful, since diffraction losses at the apertures would considerably reduce the efficiency of the disk laser, the apertures would have to be exactly centered, their diameters would have to be precisely adjusted to the resonator modes and they would also have to be cooled at least for high laser powers. The non-pumped outer region of the disk laser crystal of quasi-three-level laser systems absorbs laser radiation and thus forms a “soft” amplification/loss stop (or gain aperture) in connection with the pumped part. Ytterbium-doped laser material (Yb:YAG, etc.) is a common material used in disk lasers, and forms a quasi-three-level laser system. In the pumped inner region of the laser crystal, the laser radiation is amplified and in the non-pumped outer region, the laser radiation is absorbed. This “soft” aperture simultaneously strongly reduces diffraction losses. The position of the amplification/loss apertures in the resonator is predetermined by the axial position of the laser crystals.
In the case of rod lasers, hard apertures in the laser radiation field can be avoided by ensuring that the rod surfaces do not act as apertures. For example, rods that are only axially pumped in one central partial volume do not act as hard apertures in the laser radiation field. As another example, bonded rods do not contribute hard apertures in the laser radiation field because bonded rods have a passive (that is, a transparent or slightly absorbing) volume that is adapted to the refractive index and disposed concentrically around the active volume. In both cases, the rods produce a “soft” amplification/loss aperture in the transition between the pumped and non-pumped crystal volume similar to the disk laser crystal.